Paper webs of this type are known per se; they are used for example as furniture films or as a flooring laminate. Strict specifications must be met here in terms of quality. For this reason, expensive melamine resins are predominantly used in practice to effect impregnation.
Decor papers are impregnated here with the melamine resin and dried by heated air. When applied with heat and pressure to the substrate, for example particleboard, the melamine resin produces a scratch-free surface and ensures that the decor paper is reliably bonded to the substrate. Significantly cheaper urea resin can be used for an initial impregnation in order to reduce costs. In this case, both sides of the impregnated web must be coated with melamine resin in order to achieve the required surface properties for the laminate and to ensure reliable bonding to the board.
Conventional drying processes do not allow the web that is impregnated with pure urea resin to be adhesively bonded to the substrate.
EP 0966641 discloses a method of impregnating and drying a paper web in which the requisite application of melamine resin is controlled based on measured values for grammage. Drying is effected by heated air and is controlled based on the final moisture content of the impregnated paper.
It is desirable as much as possible to substitute is significantly cheaper urea resin for the melamine resin to impregnate paper webs.
WO 2008/134823 describes a method of making impregnated paper in which urea resin is used for the impregnation. near-infrared radiant heat is used for drying such that crosslinking of the resin is largely prevented. When the film impregnated with the urea resin is pressed against the substrate, the low level of crosslinking ensures reliable adhesive bonding. One disadvantage of the method is that it does not provide the requisite operational reliability. The method furthermore requires the use of expensive electrical energy. In the case of variegated patterns, areas of different colors dry differently due to variation in absorption and reflection, with the result that the overall web has a nonuniform moisture content.